The invention relates to the fabrication of semiconductor devices, such as dynamic random access memory devices, and more particularly to patterning and fabrication of critical size tolerance structures, such as wordlines.
In the fabrication of semiconductor integrated circuits optical pattern transfer tools are used to produce an optical pattern on photoresist which is a photosensitive material which is generally baked subsequent to its deposition on a semiconductor substrate structure. Light is directed at the optical pattern transfer tool. Transparent portions of the optical pattern transfer tool transmit the light to selected portions of the photoresist while opaque portions of the optical. pattern transfer tool prohibit the light from reaching the remaining portions of the photoresist.
When negative resist is used, photoresist not exposed to light is removed to expose portions of the substrate structure while the remaining resist protects unexposed portions of the substrate structure during subsequent processing steps. The opposite is true when positive resist is used. In this case the photo resist exposed to the light is removed. During a development step the portions to be removed are usually rendered soluble in a base solution and rinsed from the semiconductor substrate structure in de-ionized (DI) water. Remaining resist may then be hardened by re-baking to ensure adhesion of the resist to the semiconductor substrate structure.
There are different optical configurations available to expose the optical pattern transfer tool. It should be noted that presently there are xe2x80x9ctwo sizesxe2x80x9d of optical pattern transfer tools, a mask and a reticle. The mask is used to pattern an entire semiconductor wafer in one exposure step. In this case the pattern for the entire wafer is represented on the optical pattern transfer tool. The reticle represents a pattern for only a portion of the entire wafer and is stepped across the wafer during exposure to light in order to pattern the entire wafer.
There are at least two important criteria of an optical pattern transfer tool which concern the engineer. First the engineer wants to be able to determine an exact size, within tolerances, of a feature having a critical dimension. Second the engineer wants to be able to define an area having a smallest width possible without being concerned with its exact dimension.
One present optical pattern transfer tool comprises two types of materials, one which transmits light, such as quartz, and one which prohibits the transfer of light, such as chrome. Using methods presently available it is possible to create a chrome region having a critical dimension falling within specified tolerances. Thus, there is a minimum width the chrome can be and still have its exact size fall within the critical dimension specification. However, using this xe2x80x9ctwo materialxe2x80x9d optical pattern transfer tool does not allow definition of very small regions on the photoresist due to the limitations of photolithography.
A xe2x80x9cone materialxe2x80x9d optical pattern transfer tool comprises a transparent material having two surfaces, where each surface lies in a plane which is parallel to a plane of the other surface. Between the planes xe2x80x9cstepsxe2x80x9d are formed. Each step is formed where the material transitions from one planar surface to the other planar surface. Optical transfer of light is substantially prohibited at the step. This results in very small areas of the photoresist not. being exposed to light, which allows definition of very small regions on the photoresist. However, using the xe2x80x9cone materialxe2x80x9d optical pattern transfer tool does not allow dimensions to be controlled with a high degree of accuracy.
Furthermore, memory density is typically limited by a minimum lithographic feature size (F) that is imposed by lithographic processes used during fabrication. For example, the present generation of high density dynamic random access memories (DRAMs), which are capable of storing 256 Megabits of data, require an area of 8F2 per bit of data. There is a need in the art to maximize active areas on a memory device as compared to field areas in order to support increased storage capacity.
Accordingly, wordlines in these higher density memory devices require minimal width and must be accurately defined to stay within critical dimensions. When small dimensions are defined on a DRAM and critical dimensions are controlled, increased memory density is supported. Therefore, there is a need for an optical transfer tool in the manufacture of semiconductor devices wherein very small regions on a photoresist can be defined and these regions can be controlled with a high degree of accuracy.
In an exemplary embodiment the invention includes a method for forming a wordline wherein light is projected onto an optical pattern transfer tool comprised of an opaque material and a transparent material having first and second thicknesses. A sidewall of the opaque material is aligned with only a portion of a sidewall of the transparent material at a transition between the first and second thicknesses of the transparent material. The optical transfer pattern tool substantially prohibits light from passing through the opaque material and the transition of the transparent material at the portion aligned With the sidewall of the opaque material to pattern a wordline on a photoresist layer overlying an active area.
In another embodiment of the invention, a computer system comprises a memory device, the memory device including an array of memory cells, each memory cell including a transistor, a capacitor, a bit contact, and addressing circuitry coupled to the array of memory cells via wordlines for accessing individual memory cells in the array of memory cells, and a read circuit coupled to the array of memory cells via bitlines for reading individual memory cells in the array of memory cells, wherein a width of the wordline is 0.15 micrometers and the distance between two adjacent wordlines is 0.2 micrometers.
In a further embodiment of the invention, a dynamic random access memory (DRAM) device comprises an array of memory cells, each memory cell including a transistor, a capacitor, a bit contact, and addressing circuitry coupled to the array of memory cells via wordlines for accessing individual memory cells in the array of memory cells, and a read circuit coupled to the array of memory cells via bitlines for reading individual memory cells in the array of memory cells, wherein a width of the wordline is 0.15 micrometers and the distance between two adjacent wordlines is 0.2 micrometers.
By using the method of the invention the shape of the chrome patterns on the optical pattern transfer tool are adjusted to get a desired shape on the wafer. For nonactive areas on a DRAM, the width of a wordline is printed as small as 0.15 micrometers and the separation between these wordlines in the nonactive areas is as small as 0.2 micrometers at 365 nanometer lithography.